Chemical removal of zinc coating from iron



2,951,768 Patented Sept. 6, 1960 CHEMICAL REMOVAL OF ZINC COATING FROM IRON Mark P. Brash, 762 Granite St., Braintree, Mass.; Ed-

mund F. Coleman, 29 Armandine St., Dorchester, ass.; and John F. McMann, 668 Sea St., Quincy,

No Drawing. Filed Oct. 9, 1956, Ser. No. 614,991 2 Claims. (Cl. 117- 37) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention concerns a chemical composition and method of using same and in particular concerns a chemical composition and method for removing zinc from galvanized steel.

In welding steel plates it is necessary that the area to be welded is clean and free from deleterious foreign material so that a weld of integrity can be formed.

Thus in welding steel plates that have been previously galvanized it is necessary to remove the zinc from the weld area and expose a totally zinc-free surface before welding is begun. The conventional method of removing such zinc is by grinding the weld area with a pneumatic grinding machine. This method of removal is unsatisfactory because it mechanically abrades and gouges the steel surface as the zinc is removed so that there results a defective weld area.

In the past chemical removal of zinc from galvanized steel was by means of acid which not only removed the zinc but attacked the steel surface. Various attempts were made to add inhibitors to the acid to slow down the reaction so that an operator could visually detect the critical moment when zinc removal was completed and before acid attack on the steel started and so stop the reaction at this moment. This method has always been unsatisfactory because human error results in the reaction being stopped either before complete removal of the .zinc or after acid attack of the steel had begun.

Our method has eliminated this difficulty by providing an exchange reaction whereby the zinc is completely removed from the steel and automatically upon completion of such removal there is deposited a copper coating which stops the reaction. Use of the hydrochloric acid modifies the exchange reaction so that there is insured a smooth, rapid and complete removal of zinc together with a thin copper coating of optimum texture.

An object of this invention is to provide a chemical composition and method of using same.

Another object is to provide a chemical composition and method to remove zinc from galvanized steel.

Another object is to provide a chemical composition and method to remove zinc from galvanized steel so that the zinc is completely removed but the steel surface is not attacked.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of examples of the composition.

The present invention encompasses the use of a water solution of cupric chloride and hydrochloric acid which is applied to the galvanized area of steel that is to be cleaned. The fol-lowing examples of the composition are illustrative Example I 2 pints water 4 pints hydrochloric acid (30% HCl in water) 40 oz. copper chloride The 2 pints of water and 4 pints of hydrochloric acid form about a 20% solution of HCl in water.

Example II 3 pints water 3 pints hydrochloric acid 40 oz. copper chloride The 3 pints of water and 3 pints of hydrochloric acid form about a 15% solution of HCl in water.

Example III 2 /2 pints water 4 pints hydrochloric acid 24 oz. copper chloride The 2 /2 pints of water and 4 pints of hydrochloric acid form about a 17% solution of HCl in water.

The proportions of cupric chloride and hydrochloric acid are critical and when used in the critical proportions we have discovered, there are produced results that differ unexpectedly from the teachings of the prior art.

When less than my critical lower limit of hydrochloric acid is used, the zinc removal is unsatisfactory. Although the reasons are not entirely clear it may be that the exchange reaction of copper and zinc is such that there is: deleterious precipitation of copper. However, when the proportion of hydrochloric acid is above my lower criti-- cal limit, there is a uniform removal of zinc from all areas of the galvanized steel until when the last zinc is: removed a smooth deposit of copper is formed in the steel. Furthermore, when less than my critical lower limit of hydrochloric acid is used, the copper does not deposit in a satisfactory plating in exchange with the zinc. However, when the proportion of hydrochloric acid is above my lower critical limit, there is a smooth exchange reaction and a smooth coating of copper deposits on the steel to furnish a superior weld area.

When less than our critical proportion of cupric chloride is used there is an unsatisfactory removal of zinc and plating of copper. Although the reasons are not entirely clear it may be due to faulty removal of the zinc, to faulty deposit of copper, or it may result from unsatisfactory evolution of hydrogen bubbles which interfere with the zinc removal and copper plating.

We have found that the hydrochloric acid should not be less than about 15% in the water solution and should not be more than about 20% in water solution. In solutions within such range we have found that sufficient copper chloride must be used to form at least about a 50% saturated solution and as high as a saturated solution. I

The solution may be applied locally in various ways such as brushing, spraying, pouring, and by the spout of an oil can. For light galvanized coatings a single application of our solution may be sufficient to remove the zinc. For thicker layers of zinc, several applications of our solution can be successively app-lied.

Obviously many modifications and variations ,of the present invention are possible in the light of the above teachings. It is therefore 0t be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

We claim:

1. The method of preparing a local area of the surface of zinc galvanized steel for welding by removal of the zinc coating from the local area only where the weld is to be made, without injury to the underlying steel which comprises maintaining solely on such local area a water solution containingb'etween about 15% and about 20% hydrochloric acid and cupric chloride in more than ap-' lying steel, which comprises maintaining solely on'such 10 local area, a water solution of hydrochloric acid and cupric chloride in which the acid is between about .15 and 20% of the solution and the cupric chloride is sufli cient to form between a 50% and a 100% saturated solution, for a period of time until all of the adherent zinc on such local area is replaced with metallic copper from said solution, whereby as rapidly as the zinc of said coating is removed it is replaced by a coating of metallic copper that prevents damage to the underlying steel by the hydrochloric acid of the solution.

References Cited in the file of this patent UNITED STATES PATENTS 548,039 Coyle Oct. 15, 1895 2,217,921 Saukaitis Oct. 15, 1940 2,410,844

Signaigo et al Nov. 12, 1946 

1. THE METHOD OF PREPARING A LOCAL AREA OF THE SURFACE OF ZINC GALVANIZED STEEL FOR WELDING BY REMOVAL OF THE ZINC COATING FROM THE LOCAL AREA ONLY WHERE THE WELD IS TO BE MADE, WITHOUT INJURY TO THE UNDERLYING STEEL WHICH COMPRISES MAINTAINING SOLELY ON SUCH LOCAL AREA A WATER SOLUTION CONTAINING BETWEEN ABOUT 15% AND ABOUT 20% HYDROCHLORIC ACID AND CUPRIC CHLORIDE IN MORE THAN APPROXIMATELY HALF OF THAT NECESSARY TO SATURATE THE SOLUTION, FOR A PERIOD OF TIME UNTIL THE ADHERENT ZINC ON THE STEEL IN SUCH LOCAL AREA IS REPLACED BY ADHERENT METALLIC COPPER. 